EXPORT_OK says that these identifiers *can* be imported only by explicit request by the user of the module.

EXPORT would say that these identifiers *will* be automatically imported, *except* if the user explicitly writes a import list.

If you keep to the habit to always explicitly say which names you want to import (which, IMO, is a good practice anyway), it doesn't make any difference whether the names are in EXPORT or in EXPORT_OK.